This invention relates to the production of moulds, and moulded articles, and in particular to the production of foundry moulds for casting metals.
One of the most promising techniques in recent years for the production of moulds and cores for use in the foundry in recent years is a process known as the SO.sub.2 process. In this method, a moulding composition comprising a granular filler and an acid-curing synthetic resin is cured by the use of a gas, such as SO.sub.2, which produces H.sub.2 SO.sub.4 in situ, to cause the resin to cure. This process is described in detail in British Patent Specification No. 14411975.
Although, in principle, the SO.sub.2 process is capable of providing very accurate and reproducable moulds and cores, at high throughput rates, in practice it has had only very limited application.
The reason for the limited practical application of the process is that it is found that, when a pattern is used repeatedly, as is necessary in a production process to achieve the necessary throughput, a build-up of a dark residue layer takes place on the surface of the pattern exposed to the moulding mixture. This build-up can take place very rapidly, so that it is not possible to use the pattern continuously to produce more than, typically, about eighty moulds. After this time, the build-up is of such a thickness that the dimensions of the mould are unacceptably altered.
Furthermore, it is found that the build-up is very difficult to remove, and previously was only removable with brush abrasives, such as wire wool. The repeated use of such abrasive cleaning agents is highly undesirable, because over a relatively short period, it results in the dimensions of the pattern, and its surface finish, being altered. In practical terms, it might be desirable to produce eight hundred or so moulded articles during a single shift, whithout subjecting the pattern to a cleaning operation. Existing techniques mean that the maximum number of times which the pattern can be used before it needs to be cleaned is very much less than this. Thus it is necessary to provide, for each shift, two sets of patterns, one of which is being cleaned, whilst the other is in use. This results in very substantially increased costs, both for the production on an additional set of tooling, and in the labour required.
Although the rapid build-up of the deposit on the pattern is undesirable, for the reasons outlined above, the deposition of small amounts of the deposit are found to be very beneficial. When a new pattern is used for the first time, before the deposit has had time to form, it is found that sand grains stick to the surface, even though release agents are applied. This can render the first ten or so articles produced from the pattern unusable. As a small amount of the deposit forms, the sand sticking is reduced considerably.
We have conducted intensive investigations to determine methods of treating the pattern in a process of the kind described above, so as to retard (but not to prevent completely), the build-up of the deposit, and so as also to enable the pattern to be cleaned with non-abrasive cleaning agent, when the build-up has reached an unacceptable level. As a consequence, we have determined that by treating the pattern with a silicone, preferably a siloxane, more preferably a polydialkyl siloxane, such as polydimethyl siloxane, it is possible to retard the build-up of the deposit considerably, and also to permit the removal of the build-up by the use of a non-abrasive cleaning agent, when this eventually becomes necessary.